Metal halide and high-pressure sodium lamps are two examples of lamps which use a discharge vessel, also known as an arc tube, made of a ceramic material. The preferred ceramic material for these arc tubes has been polycrystalline alumina (PCA). The robustness of the ceramic material permits the use of more corrosive chemical fills and allows the discharge vessels to be operated at higher temperatures. A sealing composition, commonly referred to as a frit, is used to form a hermetic seal between the feedthrough sections of the electrodes and the ceramic body of the discharge vessel. The sealing composition may also be used to join together the ceramic pieces which form the discharge vessel. The sealing composition is designed to have a lower melting point than the ceramic components so that the seals may be formed without additional sintering/shrinkage of the ceramic vessel. The sealing composition must be resistant to attack by the chemical fill while simultaneously withstanding the high operating temperatures of the discharge vessel.
An example of the application of a sealing composition in a ceramic metal halide lamp is shown in FIG. 1. The metal halide lamp has a ceramic discharge vessel 1 which is illustrated in cross section. The discharge vessel 1 is a two-piece design which is made by joining two identically molded ceramic halves in their green state and then subjecting the green piece to a high temperature sintering. The method of making the discharge vessel typically leaves a cosmetic seam 5 in the center where the two halves were mated. A more detailed description of a method of making this type of ceramic discharge vessel is described in U.S. Pat. No. 6,620,272 which is incorporated herein by reference. The discharge vessel is usually composed of translucent polycrystalline alumina, although other ceramic materials may be used.
In this example, the discharge vessel has hemispherical end wells 17a, 17b and is commonly referred to as a bulgy shape. The bulgy shape is preferred because it provides a more uniform temperature distribution compared to right-cylinder shapes such as those described in U.S. Pat. Nos. 5,424,609 and 6,525,476. The bulgy-shaped vessel has an axially symmetric body 6 which encloses a discharge chamber 12. Two opposed capillary tubes 2 extend outwardly from the body 6 along a central axis. In this 2-piece design, the capillary tubes have been integrally molded with the body of the discharge vessel.
Electrode assemblies 14 are inserted into each capillary tube 2. One end of the electrode assemblies 14 protrudes out of an end of the capillary tube to provide an electrical connection. The tips of the electrode assemblies which extend into the discharge chamber are fitted with a tungsten coil 3 or other similar means for providing a point of attachment for the arc discharge. The electrode assemblies are sealed hermetically to the capillary tubes by sealing composition 9. During lamp operation, the electrode assemblies act to conduct an electric current from an external source of electric power to the interior of the discharge vessel in order to form an arc in the discharge chamber.
The discharge chamber 12 contains a buffer gas, e.g., 30 to 300 torr Xe or Ar, and a metal halide fill 8 which may be combined with a small amount of mercury. For example, a commercial 4200K lamp may contain mercury plus a mixture of NaI, CaI2, DyI3, HoI3, TmI3, and TlI. In general, iodide salts are more favored than fluorides because of their lower reactivity and are more favored than chlorides or bromides because they tend to be less stable at higher temperatures. However, even the iodide salts are corrosive with respect to some lamp materials especially at high temperatures. In particular, the conventional sealing composition 9 is a Al2O3—SiO2—Dy2O3 frit. The purpose of the long capillary tubes 2 is to keep the seal region cooler and reduce the corrosive action of the metal halide salts on the Al2O3—SiO2—Dy2O3 frit. The elongated capillaries however impose size limitations on the discharge lamp which in turn restrict the types of lamp configurations which may be used. Therefore, it would be advantageous to have a sealing composition which would be less affected by metal halide fills thereby permitting shortened capillaries to be used.